Cutting tools for chip forming machining of metals, such as round tools, i.e. end mills, drills, etc, and inserts, made of durable materials, such as cemented carbide, cermet, cubic boronitride or high speed steel, commonly have a wear resistant coating to prolong service life of the cutting tool. The wear resistant coatings are frequently coated using CVD since this technique has several advantages. It enables large throughput in production of the cutting tools, conformal coating on complex geometries and can readily be used to deposit insulating coating layers such as alumina.
In particular, cemented carbide cutting tools for turning are usually coated with CVD coatings comprising a layered structure of different materials to provide sufficient wear resistance, where composition, microstructure, texture etc. of the individual layers are chosen to improve certain properties of the coating for a specific application. The predominant coating used today comprises a Ti-based layer, hereinafter referred to as Ti(C,N,O) layer comprising one or more layers selected from titanium carbide, titanium nitride, titanium carbonitride, titanium oxycarbide and titanium oxycarbonitride, hereinafter referred to as (TiC, TiN, Ti(C,N), Ti(C,O), Ti(C,N,O) layers, deposited on a surface of the substrate and an alumina layer, hereinafter referred to as Al2O3 layer, deposited on the Ti(C,N,O) layer. Moderate temperature CVD (MTCVD) processes has proven to be advantageous for deposition of Ti(C,N) layers as compared to high temperature CVD (HTCVD) processes.
Larsson and Ruppi, Thin Solid Films 402 (2002) 203-210 discloses a study on the microstructure and properties of Ti(C,N) coatings deposited on cutting tool substrates using MTCVD, as compared with Ti(C,N) coatings deposited using HTCVD. The HTCVD Ti(C,N) coating exhibits equiaxed grains without preferred growth direction and an average grain size of less than 0.2 μm. In contrast the MTCVD Ti(C,N) coatings exhibit a relatively large TC(422) value in X-ray diffraction measurement, hereinafter referred to as a (422) texture, and columnar grains having a width of about 0.5 μm. The difference in microstructure is assigned to the lower temperature and aggressive precursors, such as acetonitrile (CH3CN). The MTCVD Ti(C,N) coating has better edge chipping resistance, but worse crater wear resistance, as compared to the HTCVD Ti(C,N) coating. However, flaking resistance still is critical for MTCVD Ti(C,N) coatings, in particular in demanding applications such as turning in nodular cast iron comprising intermittent cutting operations.
EP 1 187 970 A1 discloses a columnar Ti(C,N) layer with a (422) texture deposited using a MTCVD process with precursors comprising acetonitrile, titanium tetrachloride, nitrogen and hydrogen, as the above MTCVD process, and in addition a hydrocarbon such as C2H4 or C3H6, which is disclosed to give high atomic ratio of carbon to the sum of carbon and nitrogen (C/C+N) contained in the columnar Ti(C,N) layer, i.e. at least 0.70, and thus a high hardness and improved wear resistance as compared to a standard acetonitrile process. The columnar Ti(C,N) layer formed using these precursors is fine-grained with an average grain width of 0.05 to 0.5 μm and has a high fracture resistance. Albeit the improved hardness the oxidation resistance of this columnar Ti(C,N) layer may be insufficient, in particular for cutting operations generating a lot of heat in the coating.